1. Field of the Invention
The present invention relates to synchronous motors, such as a stepping motor, and a sensor-free system for controlling such a motor. Such a control system uses the back-electromotive forces generated in the motor coils in order to provide information indicating the position of the motor.
2. Discussion of the Related Art
Most of the sensor-free control systems are used with three-phase motors that are bipolar motors, that is, whose coils conduct currents in both directions.
The European application 0,433,219 describes a control system for detecting back-electromotive forces, applied to a stepping bipolar motor to provide a self-switching of the motor. Stepping motors are two-phased. Generally, self-switching of a motor consists in providing a new rotation order (to impose a new step) to the motor as soon as the preceding order (the step) has been implemented.
A drawback of the use of a bipolar motor, as is apparent in European application 0,433,219, is that such a motor requires two current inverters (or four switches) per coil, which renders the control circuit complex and expensive (8 switches are required in application 0,433,219).
Another drawback of the control system according to European application 0,433,219 is that the connection of the motor's coils is specially designed and requires the use of a motor with eight supply wires. 8-wire motors are not frequently used and are therefore seldom available as standard elements.
A further drawback of the system according to European application 0,433,219 is that, during the recovery phases during which the currents in the coils are reversed (at each step switching), the voltages measured by the system are difficult to use and can cause unforeseeable reactions of the system.
The control of a one-pole synchronous motor (i.e., a motor whose coils conduct currents in one direction only) requires one switch per coil. There exist sensor-free control systems for one-pole motors, but they are based, like the three-phase motors, on a discontinuous power supply of the coils and therefore do not allow to take full advantage of the performances (torque, efficiency) of the motor.